1. Field of the Invention
The present invention generally relates to data switching devices, and more particularly a data switching device which employs a quantity-dependent charging method in which the charge is accounted based on the quantity of user information transferred between subscribers. More specifically, the present invention is concerned with a high-speed relay network such as a frame relay switching network using a permanent virtual connection (PVC) link or a cell relay switching network (an asynchronous transfer mode (ATM) network) in which the charge is accounted based on the quantity of user information transferred between the subscribers. Furthermore, the present invention relates to a method of notifying the subscriber of the charge at a given timing after the communication is completed.
Recently, there has been an increasing demand for data switching networks and a requirement of speeding up a data transfer. In such a situation, there has been a considerable activity in the research and development of a high-speed relay switching network which should be replaced by a conventional packet switching network.
Currently working high-speed relay switching networks such as a frame relay switching network and a cell relay switching network provide PVC services in which a link (PVC link) is permanently established between specific subscribers. In the PVC link, a virtual pass identifier (VPI) and a virtual connection identifier (VCI) are fixed. The charge accounted by transferring data via a PVC link is fixed irrespective of the quantity of data actually transferred. There is another charge accounting method based on the quantity of data actually transferred. Such a charge accounting method is called quantity-dependent charge accounting.
In the conventional high-speed relay switching networks, there is no way to immediately notify the sender subscriber of the charge for the communication just completed. Usually, the subscriber is notified of the total charge at a given interval. Hence, it is desired that the subscriber is immediately notified of the charge for the just completed communication.
2. Description of the Prior Art
FIG. 1 is a block diagram of a system having a frame relay network 1 to which frame relay terminals 3A and 3B are connected. PVC links are formed between the frame relay terminals 3A and 3B. For example, a PVC link is formed between the terminal A and B in which there are formed a link identified by DLCI (Data Link Connection Identifier) "i" of a physical line between the terminal A and the network 1 and a link identified by DLCI "m" of a physical line between the terminal B and the network 1.
FIG. 2 is a diagram showing a conventional charge accounting system in the system shown in FIG. 1. The frame relay network 1 includes frame relay switches 9a and 9b. The sender terminal A and the receiver terminal D are connected via a link formed by the frame relay switches 9a and 9b. It should be noted that a SVC (Switched Virtual Connection) link is also employed in the conventional high-speed relay networks. The SVC link is established by specifying a destination subscriber each time a sender subscriber starts to a communication with the destination subscriber. On the other hand, in a communication via the PVC link, a destination subscriber is known before starting the communication.
In FIG. 2, the terminals A and D can communicate with either a SVC link or a PVC link. The charge accounting is carried out in the frame relay switch 9a directly connected to the sender terminal A. Information concerning charge accounting (the quantity of data transferred to the destination terminal D) is transferred from the frame relay switch 9b to the frame relay switch 9a by an in-network control signal indicated by a broken line shown in FIG. 2.
According to the UNI (User Network Interface) defined by Recommendation ITU-T COM 7-R 33-E (the disclosure of which is hereby incorporated by reference), a disconnect message is defined for a protocol of the SVC link. The disconnect message can contain information concerning the charge accounting. Hence, the sender subscriber A can immediately be notified of the charge for the communication by the disconnect message.
On the other hand, a disconnect message is not defined in a protocol of the PVC link. Hence, the sender subscriber A using the PVC link do not have any way to receive information concerning the charge for a communication immediately after the communication is completed.